Sand the surface of the prefabricated fireplace using medium-grit sandpaper, which will remove any areas that are uneven or rough.
Brush off any dust and debris from the fireplace caused by the sandpaper using a 2-inch flat paintbrush.
Spread a 1/2-inch thick layer of fine grout over the entire surface of the prefab fireplace where you want the stones placed using a trowel.
Run a grout rake over the surface of the grout-covered fireplace to scratch or roughen up the surface. This enables the stone tiles to adhere more readily onto the fireplace.
Press 1/4-inch thick stone tiles halfway into the grout-covered surface, spacing the stone tiles 1/4-inch apart. The excess grout will be pushed up in between the individual stone tiles.
Scrape the trowel over the edges of the stone tiles to remove the excess grout and to create smooth creases.
Allow the stone prefabricated fireplace to dry for 12 to 18 hours before use.